DC-DC converters transfer power from a DC electrical input source to a load by transferring energy between windings of an isolation transformer. The DC output voltage delivered to the load is controlled by adjusting the timing of internal power switching elements (e.g., by controlling the converter switching frequency and/or the switch duty cycle and/or the phase of switches). As defined herein, the functions of a “DC-DC converter” comprise: a) isolation between the input source and the load; b) conversion of an input voltage to an output voltage; and c) regulation of the output voltage. DC-DC converters may be viewed as a subset of a broad class of switching power converters, referred to as “switching regulators,” which convert power from an input source to a load by processing energy through intermediate storage in reactive elements. As defined herein, the functions of a “Switching Regulator” comprise: a) conversion of an input voltage to an output voltage, and b) regulation of the output voltage. If the required output voltage is essentially a positive or negative integer (or rational) multiple of the input voltage, the conversion function may also be efficiently performed by a capacitive “Charge Pump,” which transfers energy by adding and subtracting charge from capacitors.
Vinciarelli et al, “Efficient Power Conversion” U.S. Pat. No. 5,786,992 disclose expanding the operating voltage range of isolated DC-DC converters by connecting their inputs and/or outputs in series.
Non-resonant full-bridge, half-bridge, and push-pull DC-to-DC transformer topologies are known. See e.g., Severns and Bloom, “Modern DC-to-DC Switchmode Power Conversion Circuits,” ISBN 0-442-21396-4, pp. 78–111. Series, parallel, and other resonant forms of switching power converters are also known. See e.g., Steigerwald, “A Comparison of Half-Bridge Resonant Converter Topologies,” IEEE Transactions on Power Electronics, Vol. 2, No. 2, April, 1988. Variable frequency, series resonant, half-bridge converters for operation from an input voltage source are described in Baker, “High Frequency Power Conversion With FET-Controlled Resonant Charge Transfer,” PCI Proceedings, April 1983, and in Nerone, U.S. Pat. No. 4,648,017. Half-bridge, single-stage, ZVS, multi-resonant, variable frequency converters, which operate from an input voltage source are shown in Tabisz et al, U.S. Pat. No. 4,841,220 and Tabisz et al, U.S. Pat. No. 4,860,184. A variable frequency, full-bridge, resonant converter, in which an inductor is interposed between the input source and the resonant converter is described in Divan, “Design Considerations for Very High Frequency Resonant Mode DC/DC Converters,” IEEE Transactions on Power Electronics, Vol. PE-2, No. 1, January, 1987. A variable frequency, ZVS, half-bridge LLC series resonant converter is described in Bo Yang et al, “LLC Resonant Converter for Front End DC-DC Conversion,” CPES Seminar 2001, Blacksburg, Va., April 2001. Analysis and simulation of a “Low Q” half-bridge series resonant converter, wherein the term “Low Q” refers to operation at light load, is described in Bo Yang et al, “Low Q Characteristic of Series Resonant Converter and Its Application,” CPES Seminar 2001, Blacksburg, Va., April 2001.
Fixed-frequency half-bridge and full-bridge resonant converters are also known in which output voltage control is achieved by controlling the relative timing of switches. A half-bridge, single-stage, ZVS, multi-resonant, fixed-frequency converter that operates from an input voltage source is shown in Jovanovic et al, U.S. Pat. No. 4,931,716. A full-bridge, single-stage, ZVS, resonant, fixed-frequency converter that operates from an input voltage source is shown in Henze et al, U.S. Pat. No. 4,855,888.
A full-bridge, single-stage, ZCS, series-resonant, fixed-frequency converter, operating at a frequency equal to the characteristic resonant frequency of the converter, is shown in Palz, “Stromversorgung von Satelliten—Wanderfeldröhren hoher Leistung” (“Power Supply for Satellites—High Capacity Traveling-Wave Tubes”), Siemens Zeitschrift, Vol. 48, 1974, pp. 840–846. Half and full-bridge, single-stage, ZVS, resonant, converters, for powering fluorescent tubes are shown in Nalbant, U.S. Pat. No. 5,615,093.
A DC-to-DC Transformer offered for sale by SynQor, Hudson, Mass., USA, called a “BusQor™ Bus Converter,” that converts a regulated 48VDC input to a 12 VDC output at a power level of 240 Watts and that can be paralleled with other similar converters for increased output power delivery, and that is packaged in a quarter brick format, is described in data sheet “Preliminary Tech Spec, Narrow Input, Isolated DC/DC Bus Converter,” SynQor Document No. 005-2BQ512J, Rev. 7, August, 2002.
The art of resonant power conversion, including operation below or above resonant frequency, utilizing either ZCS or ZVS control techniques and allowing the resonant cycle to be either completed or purposely interrupted, is summarized in Chapter 19 of Erickson and Maksimovic, “Fundamentals of Power Electronics,” 2nd Edition, Kluwer Academic Publishers, 2001.
Cascaded converters, in which a first converter is controlled to generate a voltage or current, which serves as the source of input power for a DC-to-DC transformer stage, are known. A discussion of canonical forms of cascaded converters is given in Severns and Bloom, ibid, at, e.g., pp. 114–117, 136–139. Baker, ibid, discusses the use of a voltage pre-regulator cascaded with a half-bridge, resonant, variable-frequency converter. Jones, U.S. Pat. No. 4,533,986 shows a continuous-mode PWM boost converter cascaded with both PWM converters and FM resonant half-bridge converters for improving holdup time and improving the power factor presented to an AC input source. A zero-voltage transition, current-fed, full-bridge PWM converter, comprising a PWM boost converter delivering a controlled current to a PWM, full-bridge converter, is shown in Hua et al, “Novel Zero-Voltage Transition PWM Converters,” IEEE Transactions on Power Electronics, Vol. 9, No. 2, March, 1994, p. 605. Stuart, U.S. Pat. No. 4,853,832, shows a full-bridge series-resonant converter cascaded with a series-resonant DC-to-DC transformer stage for providing AC bus power to distributed rectified loads. A half-bridge PWM DC-to-DC transformer stage for use in providing input power to point-of-load DC-DC converters in a DPA is described in Mweene et al, “A High-Efficiency 1.5 kW, 390–50V Half-Bridge Converter Operated at 100% Duty Ratio,” APEC '92 Conference Proceedings, 1992, pp. 723–730. Schlecht, U.S. Pat. Nos. 5,999,417 and 6,222,742 shows DC-DC converters which incorporate a DC-to-DC transformer stage cascaded with a switching regulator. Vinciarelli, “Buck-Boost DC-DC Switching Power Conversion,” U.S. patent application Ser. No. 10/214,859, filed Aug. 8, 2002, assigned to the same assignee as this application and incorporated by reference, discloses a new, high efficiency, ZVS buck-boost converter topology and shows a front-end converter comprising the disclosed topology cascaded with a DC-DC converter and a DC-to-DC transformer.